270 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
ation of Species as Related to their Geographical Distribution, illus- 
trated by the Achatinelline,” was published in Nature for July 18, 1872; 
the other, entitled ‘* Diversity of Evolution under One Set of External 
Conditions,” after being read before the British Association for the 
Advancement of Science in August, 1872, was, through the kindness 
of Mr. Alfred Wallace, brought before the Linnean Society, and was 
finally published in the Linnean Society’s Journal, Zodlogy, vol. X1, pp. 
496-505. 
In the former paper I used the following words in calling attention 
to the impossibility of explaining the origin and distribution of these 
forms by natural selection: “ Whether we call the different forms 
species or varieties, the same questions are suggested as to how they 
have arisen and as to how they have been distributed in their several 
localities. In answering these questions, we find it difficult to point to 
any of those active causes of accumulated variation, classed by Darwin 
as natural selection. - - - There is no reason to doubt that some 
varieties less fitted to survive have disappeared; but it does not fol- 
low that the ‘survival of the fittest’ (those best fitted when compared 
with those dying prematurely, but equally fitted when compared with 
‘ach other) is the determining cause which has led to these three 
species being separated from each other in adjoining valleys. The ‘sur- 
vival of the fittest’ still leaves a problem concerning the distribution of those 
equally fitted. It can not be shown that the ‘survival of the fittest’ is 
at variance with the survival, under one set of external circumstances, 
of varieties differing more and more widely from each other in each 
successive generation. The case of the species under consideration 
does not seem to be one in which difference of environment has been 
the occasion of different forms being preserved in the different locali- 
ties. It is rather one in which varieties resulting from some other 
cause, though equally fitted to survive in each of the localities, have 
been distributed according to their affinities in separate localities.” 
In the latter paper I raised the following questions concerning nat- 
ural selection. ‘The terms ‘natural selection’ and ‘survival of the 
fittest’ - - - imply that there are variations that may be accumu- 
lated according to the differing demands of external conditions. 
What, then, is the effect of these variations when the external condi- 
tions remain the same? Or can it be shown that there is no change in 
organisms that is not the result of change in external conditions? 
Again, if the initiation of change in the organism is through change in 
the environment, - - - does the change expend itself in producing 
from each species just one new species completely fitted to the conditions, 
or may it produce from one stock many that are equally fitted?” (p. 497). 
In answering these questions I called “attention to the variation and 
distribution of terrestrial mollusks, more especially those found on the 
Sandwich Islands,” and gave what seemed to me strong reasons for 
belieying that ‘the evolution of these different forms can not be attrib- 
